Congestive Heart Failure (“CHF”) is a condition that occurs when the heart becomes damaged and reduces blood flow to the organs of the body. If blood flow decreases sufficiently, kidney function becomes impaired and results in fluid retention, abnormal hormone secretions and increased constriction of blood vessels. These results increase the workload of the heart and further decrease the capacity of the heart to pump blood through the kidney and circulatory system.
This reduced capacity further reduces blood flow to the kidney. It is believed that progressively decreasing perfusion of the kidney is a principal non-cardiac cause perpetuating the downward spiral of CHF. Moreover, the fluid overload and associated clinical symptoms resulting from these physiologic changes are predominant causes for excessive hospital admissions, terrible quality of life and overwhelming costs to the health care system due to CHF.
While many different diseases may initially damage the heart, once present, CHF is split into two types: Chronic CHF and Acute (or Decompensated-Chronic) CHF. Chronic Congestive Heart Failure is a longer term, slowly progressive, degenerative disease. Over years, chronic congestive heart failure leads to cardiac insufficiency. Chronic CHF is clinically categorized by the patient's ability to exercise or perform normal activities of daily living (such as defined by the New York Heart Association Functional Class). Chronic CHF patients are usually managed on an outpatient basis, typically with drugs.
Chronic CHF patients may experience an abrupt, severe deterioration in heart function, termed Acute Congestive Heart Failure, resulting in the inability of the heart to maintain sufficient blood flow and pressure to keep vital organs of the body alive. These Acute CHF deteriorations can occur when extra stress (such as an infection or excessive fluid overload) significantly increases the workload on the heart in a stable chronic CHF patient. In contrast to the stepwise downward progression of chronic CHF, a patient suffering acute CHF may deteriorate from even the earliest stages of CHF to severe hemodynamic collapse. In addition, Acute CHF can occur within hours or days following an Acute Myocardial Infarction (“AMI”), which is a sudden, irreversible injury to the heart muscle, commonly referred to as a heart attack.
As mentioned, the kidneys play a significant role in the progression of CHF, as well as in Chronic Renal Failure (“CRF”), End-Stage Renal Disease (“ESRD”), hypertension (pathologically high blood pressure) and other cardio-renal diseases. The functions of the kidney can be summarized under three broad categories: filtering blood and excreting waste products generated by the body's metabolism; regulating salt, water, electrolyte and acid-base balance; and secreting hormones to maintain vital organ blood flow. Without properly functioning kidneys, a patient will suffer water retention, reduced urine flow and an accumulation of waste toxins in the blood and body. These conditions resulting from reduced renal function or renal failure (kidney failure) are believed to increase the workload of the heart. In a CHF patient, renal failure will cause the heart to further deteriorate as the water build-up and blood toxins accumulate due to the poorly functioning kidneys and, in turn, cause the heart further harm.
The primary functional unit of the kidneys that is involved in urine formation is called the “nephron.” Each kidney consists of about one million nephrons. The nephron is made up of a glomerulus and its tubules, which can be separated into a number of sections: the proximal tubule, the medullary loop (loop of Henle), and the distal tubule. Each nephron is surrounded by different types of cells that have the ability to secrete several substances and hormones (such as renin and erythropoietin). Urine is formed as a result of a complex process starting with the filtration of plasma water from blood into the glomerulus. The walls of the glomerulus are freely permeable to water and small molecules but almost impermeable to proteins and large molecules. Thus, in a healthy kidney, the filtrate is virtually free of protein and has no cellular elements. The filtered fluid that eventually becomes urine flows through the tubules. The final chemical composition of the urine is determined by the secretion into, and re-absorption of substances from, the urine required to maintain homeostasis.
Receiving about 20% of cardiac output, the two kidneys filter about 125 ml of plasma water per minute. Filtration occurs because of a pressure gradient across the glomerular membrane. The pressure in the arteries of the kidney pushes plasma water into the glomerulus causing filtration. To keep the Glomerulur Filtration Rate (“GFR”) relatively constant, pressure in the glomerulus is held constant by the constriction or dilatation of the afferent and efferent arterioles, the muscular walled vessels leading to and from each glomerulus.
In a CHF patient, the heart will progressively fail, and blood flow and pressure will drop in the patient's circulatory system. During acute heart failure, short-term compensations serve to maintain perfusion to critical organs, notably the brain and the heart that cannot survive prolonged reduction in blood flow. However, these same responses that initially aid survival during acute heart failure become deleterious during chronic heart failure.
A combination of complex mechanisms contribute to deleterious fluid overload in CHF. As the heart fails and blood pressure drops, the kidneys cannot function and become impaired due to insufficient blood pressure for perfusion. This impairment in renal function ultimately leads to the decrease in urine output. Without sufficient urine output, the body retains fluids, and the resulting fluid overload causes peripheral edema (swelling of the legs), shortness of breath (due to fluid in the lungs), and fluid retention in the abdomen, among other undesirable conditions in the patient.
In addition, the decrease in cardiac output leads to reduced renal blood flow, increased neurohormonal stimulus, and release of the hormone renin from the juxtaglomerular apparatus of the kidney. This results in avid retention of sodium and, thus, volume expansion. Increased renin results in the formation of angiotensin, a potent vasoconstrictor. Heart failure and the resulting reduction in blood pressure also reduce the blood flow and perfusion pressure through organs in the body other than the kidneys. As they suffer reduced blood pressure, these organs may become hypoxic, resulting in a metabolic acidosis that reduces the effectiveness of pharmacological therapy and increases a risk of sudden death.
This spiral of deterioration that physicians observe in heart failure patients is believed to be mediated, at least in part, by activation of a subtle interaction between heart function and kidney function, known as the renin-angiotensin system. Disturbances in the heart's pumping function results in decreased cardiac output and diminished blood flow. The kidneys respond to the diminished blood flow as though the total blood volume was decreased, when in fact the measured volume is normal or even increased. This leads to fluid retention by the kidneys and formation of edema, thereby causing the fluid overload and increased stress on the heart
Systemically, CHF is associated with an abnormally elevated peripheral vascular resistance and is dominated by alterations of the circulation resulting from an intense disturbance of sympathetic nervous system function. Increased activity of the sympathetic nervous system promotes a downward vicious cycle of increased arterial vasoconstriction (increased resistance of vessels to blood flow) followed by a further reduction of cardiac output, causing even more diminished blood flow to the vital organs.
In CHF via the previously explained mechanism of vasoconstriction, the heart and circulatory system dramatically reduce blood flow to the kidneys. During CHF, the kidneys receive a command from higher neural centers via neural pathways and hormonal messengers to retain fluid and sodium in the body. In response to stress on the heart, the neural centers command the kidneys to reduce their filtering functions. While in the short term, these commands can be beneficial, if these commands continue over hours and days they can jeopardize the person's life or make the person dependent on artificial kidney for life by causing the kidneys to cease functioning.
When the kidneys do not fully filter the blood, a huge amount of fluid is retained in the body, which results in bloating (fluid retention in tissues) and increases the workload of the heart. Fluid can penetrate into the lungs, and the patient becomes short of breath. This odd and self-destructive phenomenon is most likely explained by the effects of normal compensatory mechanisms of the body that improperly perceive the chronically low blood pressure of CHF as a sign of temporary disturbance, such as bleeding.
In an acute situation, the body tries to protect its most vital organs, the brain and the heart, from the hazards of oxygen deprivation. Commands are issued via neural and hormonal pathways and messengers. These commands are directed toward the goal of maintaining blood pressure to the brain and heart, which are treated by the body as the most vital organs. The brain and heart cannot sustain low perfusion for any substantial period of time. A stroke or a cardiac arrest will result if the blood pressure to these organs is reduced to unacceptable levels. Other organs, such as the kidneys, can withstand somewhat longer periods of ischemia without suffering long-term damage. Accordingly, the body sacrifices blood supply to these other organs in favor of the brain and the heart.
The hemodynamic impairment resulting from CHF activates several neurohormonal systems, such as the renin-angiotensin and aldosterone system, sympatho-adrenal system and vasopressin release. As the kidneys suffer from increased renal vasoconstriction, the GFR drops, and the sodium load in the circulatory system increases. Simultaneously, more renin is liberated from the juxtaglomerular of the kidney. The combined effects of reduced kidney functioning include reduced glomerular sodium load, an aldosterone-mediated increase in tubular reabsorption of sodium, and retention in the body of sodium and water. These effects lead to several signs and symptoms of the CHF condition, including an enlarged heart, increased systolic wall stress, an increased myocardial oxygen demand, and the formation of edema on the basis of fluid and sodium retention in the kidney. Accordingly, sustained reduction in renal blood flow and vasoconstriction is directly responsible for causing the fluid retention associated with CHF.
CHF is progressive, and as of now, not curable. The limitations of drug therapy and its inability to reverse or even arrest the deterioration of CHF patients are clear. Surgical therapies are effective in some cases, but limited to the end-stage patient population because of the associated risk and cost. Furthermore, the dramatic role played by kidneys in the deterioration of CHF patients is not adequately addressed by current surgical therapies.
The autonomic nervous system is recognized as an important pathway for control signals that are responsible for the regulation of body functions critical for maintaining vascular fluid balance and blood pressure. The autonomic nervous system conducts information in the form of signals from the body's biologic sensors such as baroreceptors (responding to pressure and volume of blood) and chemoreceptors (responding to chemical composition of blood) to the central nervous system via its sensory fibers. It also conducts command signals from the central nervous system that control the various innervated components of the vascular system via its motor fibers.
Experience with human kidney transplantation provided early evidence of the role of the nervous system in kidney function. It was noted that after transplant, when all the kidney nerves were totally severed, the kidney increased the excretion of water and sodium. This phenomenon was also observed in animals when the renal nerves were cut or chemically destroyed. The phenomenon was called “denervation diuresis” since the denervation acted on a kidney similar to a diuretic medication. Later the “denervation diuresis” was found to be associated with vasodilatation of the renal arterial system that led to increased blood flow through the kidney. This observation was confirmed by the observation in animals that reducing blood pressure supplying the kidneys reversed the “denervation diuresis.”
It was also observed that after several months passed after the transplant surgery in successful cases, the “denervation diuresis” in transplant recipients stopped and the kidney function returned to normal. Originally, it was believed that the “renal diuresis” was a transient phenomenon and that the nerves conducting signals from the central nervous system to the kidney were not essential to kidney function. Later discoveries suggested that the renal nerves had a profound ability to regenerate and that the reversal of “denervation diuresis” could be attributed to the growth of new nerve fibers supplying the kidneys with necessary stimuli.
Another body of research focused on the role of the neural control of secretion of the hormone renin by the kidney. As was discussed previously, renin is a hormone responsible for the “vicious cycle” of vasoconstriction and water and sodium retention in heart failure patients. It was demonstrated that an increase or decrease in renal sympathetic nerve activity produced parallel increases and decreases in the renin secretion rate by the kidney, respectively.
In summary, it is known from clinical experience and the large body of animal research that an increase in renal sympathetic nerve activity leads to vasoconstriction of blood vessels supplying the kidney, decreased renal blood flow, decreased removal of water and sodium from the body, and increased renin secretion. It is also known that reduction of sympathetic renal nerve activity, e.g., via denervation, may reverse these processes.
It has been established in animal models that the heart failure condition results in abnormally high sympathetic stimulation of the kidney. This phenomenon was traced back to the sensory nerves conducting signals from baroreceptors to the central nervous system. Baroreceptors are present in the different locations of the vascular system. Powerful relationships exist between baroreceptors in the carotid arteries (supplying the brain with arterial blood) and sympathetic nervous stimulus to the kidneys. When arterial blood pressure was suddenly reduced in experimental animals with heart failure, sympathetic tone increased. Nevertheless, the normal baroreflex likely is not solely responsible for elevated renal nerve activity in chronic CHF patients. If exposed to a reduced level of arterial pressure for a prolonged time, baroreceptors normally “reset,” i.e., return to a baseline level of activity, until a new disturbance is introduced. Therefore, it is believed that in chronic CHF patients, the components of the autonomic-nervous system responsible for the control of blood pressure and the neural control of the kidney function become abnormal. The exact mechanisms that cause this abnormality are not fully understood, but its effects on the overall condition of the CHF patients are profoundly negative.
End-Stage Renal Disease is another condition at least partially controlled by renal neural activity. There has been a dramatic increase in patients with ESRD due to diabetic nephropathy, chronic glomerulonephritis and uncontrolled hypertension. Chronic Renal Failure slowly progresses to ESRD. CRF represents a critical period in the evolution of ESRD. The signs and symptoms of CRF are initially minor, but over the course of 2-5 years, become progressive and irreversible. While some progress has been made in combating the progression to, and complications of, ESRD, the clinical benefits of existing interventions remain limited.
It has been known for several decades that renal diseases of diverse etiology (hypotension, infection, trauma, autoimmune disease, etc.) can lead to the syndrome of CRF characterized by systemic hypertension, proteinuria (excess protein filtered from the blood into the urine) and a progressive decline in GFR ultimately resulting in ESRD. These observations suggest that CRF progresses via a common pathway of mechanisms and that therapeutic interventions inhibiting this common pathway may be successful in slowing the rate of progression of CRF irrespective of the initiating cause.
To start the vicious cycle of CRF, an initial insult to the kidney causes loss of some nephrons. To maintain normal GFR, there is an activation of compensatory renal and systemic mechanisms resulting in a state of hyperfiltration in the remaining nephrons. Eventually, however, the increasing numbers of nephrons “overworked” and damaged by hyperfiltration are lost. At some point, a sufficient number of nephrons are lost so that normal GFR can no longer be maintained. These pathologic changes of CRF produce worsening systemic hypertension, thus high glomerular pressure and increased hyperfiltration. Increased glomerular hyperfiltration and permeability in CRF pushes an increased amount of protein from the blood, across the glomerulus and into the renal tubules. This protein is directly toxic to the tubules and leads to further loss of nephrons, increasing the rate of progression of CRF. This vicious cycle of CRF continues as the GFR drops with loss of additional nephrons leading to further hyperfiltration and eventually to ESRD requiring dialysis. Clinically, hypertension and excess protein filtration have been shown to be two major determining factors in the rate of progression of CRF to ESRD.
Though previously clinically known, it was not until the 1980s that the physiologic link between hypertension, proteinuria, nephron loss and CRF was identified. In the 1990s the role of sympathetic nervous system activity was elucidated. Afferent signals arising from the damaged kidneys due to the activation of mechanoreceptors and chemoreceptors stimulate areas of the brain responsible for blood pressure control. In response, the brain increases sympathetic stimulation on the systemic level, resulting in increased blood pressure primarily through vasoconstriction of blood vessels. When elevated sympathetic stimulation reaches the kidney via the efferent sympathetic nerve fibers, it produces major deleterious effects in two forms. The kidneys are damaged by direct renal toxicity from the release of sympathetic neurotransmitters (such as norepinephrine) in the kidneys independent of the hypertension. Furthermore, secretion of renin that activates Angiotensin II is increased, which increases systemic vasoconstriction and exacerbates hypertension.
Over time, damage to the kidneys leads to a further increase of afferent sympathetic signals from the kidney to the brain. Elevated Angiotensin II further facilitates internal renal release of neurotransmitters. The feedback loop is therefore closed, which accelerates deterioration of the kidneys.
In view of the foregoing, it would be desirable to provide methods and apparatus for the treatment of congestive heart failure, renal disease, hypertension and/or other cardio-renal diseases via renal neuromodulation and/or denervation.